Inputs data from external and internal files.
Syntax
The syntax of the READ statement can take one of the following forms:
Long form (for use when reading from
a connected file):
READ (io-specifier-list) [input-list] |
Short form (for use when reading from standard input):
READ format [, input-list] |
Short namelist-directed form (for use when reading
from standard input into a namelist group):
- io-specifier-list
is a list of the following comma-separated I/O specifiers:
- [UNIT=]unit
specifies the unit
connected to the input file. unit can be one of the following:
The name of a character variable,
indicating an internal file
An integer expression that evaluates to the unit
connected to an external file
An asterisk, indicating a pre-connection to unit
5 (standard input)
If the optional keyword UNIT= is omitted, unit must be the first item in io-specifier-list.
- [FMT=]format
specifies the format
specification for formatting the data. format can be one of the following:
An asterisk (*), specifying list-directed I/O.
The label of a FORMAT statement containing the format specification.
An integer variable that has been assigned the label
of a FORMAT statement.
A character expression that provides the format
specification.
If the optional keyword FMT= is omitted, format must be the second item in io-specifier-list.
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| |
|
 | NOTE: The NML= and FMT= specifier may not both appear in the same
io-specifier-list. |
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|
- [NML=]name
specifies the name
of a namelist group for namelist-directed input. name must have been defined in a NAMELIST statement. If the optional keyword NML= is omitted, name must be the second item in the list. The first item must
be the unit specifier without the optional keyword UNIT=.
The NML= and FMT= specifier may not both appear in the same io-specifier-list.
- ADVANCE=character-expression
specifies whether
to use advancing I/O for this statement. character-expression can be one of the following arguments:
Table 10-47 Title not available (READ)
| 'YES' | Use
advancing formatted sequential I/O (default). |
| 'NO' | Use nonadvancing formatted sequential I/O. |
If the ADVANCE= specifier appears in io-specifier-list, unit must be connected to an external file opened for
formatted sequential I/O. Also, ADVANCE='NO' must be specified if the EOR= or SIZE= specifier appear in the list. Nonadvancing I/O
is incompatible with list-directed and namelist I/O.
- END=stmt-label
specifies the label
of the executable statement to which control passes if an end-of-file
record is encountered. This specifier is only valid for reading
files opened for sequential access.
- EOR=stmt-label
specifies the label
of the executable statement to which control passes if an end-of-record
condition is encountered. This specifier may appear in io-specifier-list only if ADVANCE='NO' also appears in the list.
- IOSTAT=integer-variable
returns the I/O
status after the statement executes. If the statement successfully
executes, integer-variable is set to zero. If an end-of-file record is encountered
without an error condition, it is set to a negative integer. If
an error occurs, integer-variable is set to a positive integer that indicates which
error occurred.
- REC=integer-expression
specifies the number
of the record to be read from a file connected for direct access.
This specifier cannot appear in io-specifier-list with the NML=, ADVANCE=, SIZE=, and EOR= specifiers, nor with FMT=* (for list-directed I/O).
- SIZE=integer-variable
returns the number
of characters that have been read by this READ statement. This specifier may appear in io-specifier-list only if ADVANCE='NO' also appears in the list.
- input-list
is a comma-separated list of data items for input.
The data items can include variables and implied-DO lists.
- format
is one of the following:
An asterisk (*), specifying list-directed I/O.
The label of a FORMAT statement containing the format specification.
An integer variable that has been assigned the label
of a FORMAT statement.
An embedded format
specification.
- name
is the name of
a namelist group, as previously defined by a NAMELIST statement. Using the namelist-directed syntax,
the READ statement takes its input from standard input.
To read from a connected file, you must use the NML= specifier with the full syntax form, as described
below.
Description
The READ statement transfers data from an external or internal
file to internal storage. An external file can be opened for sequential
access or direct access. If it is opened for sequential access,
the READ statement can perform the following types of I/O:
If the file is opened for direct access, the READ statement can perform formatted or unformatted
I/O.
READ statements operating on internal files can perform
formatted or list-directed I/O.
Examples
The following examples illustrate different uses of the READ statement.
Formatted sequential I/O
The following READ statement reads 10 formatted records from a file
opened for sequential access, using an implied-DO list to read the data into the array x_array. If the end-of-file record is encountered before
the array is filled, execution control passes to the statement at label
99.
READ (41, '(F10.2)', END=99) (x_array(i),i=1,10) |
Nonadvancing I/O
The following READ statement takes its input from a file that was
opened for sequential access and is connected to unit 9. It uses
nonadvancing I/O to read an integer into the variable key. If the statement encounters the end-of-record
condition before it can complete execution, control will pass to
the executable statement at label 100. After the statement executes, the number of characters
that have been read will be stored in cnt.
INTEGER :: key
READ (UNIT=9, '(I4)', ADVANCE='NO', SIZE=cnt, EOR=100) key |
Internal file
The following statement
inputs a string of characters from the internal file cfile, uses an embedded format specification to perform
format conversion, and stores the results in the variables i and x:
READ (cfile, FMT='(I5, F10.5)') i, x |
Namelist-directed I/O
Each of the four READ statements in the next example uses a different
style of syntax to do exactly the same thing:
NAMELIST /nl/ a, b, c
READ (UNIT=5, NML=nl) ! 5 = standard input
READ (5, nl)
READ (*, NML=nl) ! * = standard input
READ nl ! assume standard input |
List-directed I/O
The following statement
takes its data from standard input, storing the converted value
in int_var. The format conversion is based on the type of int_var.
If you knew the format, you could substitute for the asterisk
one of the following:
The label of the FORMAT statement with the format specification, as in
the following:
READ 100, int_var
100 FORMAT(I4) |
An embedded format specification, as in the following:
Unformatted direct-access I/O
The following statement
takes its input from the file connected to unit 31. The REC= specifier indicates that the file has been opened
for direct access and that this statement will read the record whose
number is stored in the variable rec_num. If an I/O error occurs during the execution of
the statement, an error number will be stored in ios, and execution control will branch to the executable
statement at label 99.
READ (31, REC=rec_num, ERR=99, IOSTAT=ios) a, b |
Related statements
CLOSE, OPEN, and WRITE.
Related concepts
For more about I/O concepts, including information about files
and different types of I/O, see Chapter 8 “I/O
and file handling”. This chapter also lists example programs that use
I/O. For information about I/O formatting, see Chapter 9 “I/O
formatting”.
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